Method for recording a holographic optical element

ABSTRACT

A fingerprint sensor having a holographic phase grating glued on one right angle surface of a prism is described where a light source oriented normally relative to the hypotenuse surface of the prism illuminates an interface of the grating and finger surface. Incident light rays are refracted and absorbed where ridges of the finger surface contact the grating surface, whereas light rays are totally internally reflected in areas between and pores in finger surface ridges not in contact the grating surface. The reflected light rays, diffracted by the holographic phase grating, propagate normally (⊥) back into the right angle prism where they are reflected normally (⊥) out the remaining right angle surface of the prism by a second total internal reflection at the internal hypotenuse surface of the prism. The reflected light rays emerging from the sensor contain high contrast, detailed images of the ridges and valleys of the finger surface including pores in the ridges, i.e., a high quality fingerprint image oriented in a plane normal to the optical axis. The holographic phase grating eliminates image distortion due dimensional compression, and there is no necessity for optically correcting astigmatism in the image before capture. Fingerprint images of a quality that allows resolution of pores on the finger surface ridges are reliably obtained making the invented fingerprint sensor ideally suited for providing input to fingerprint recording, recognition and verification systems.

RELATED APPLICATIONS

This application is division of application Ser. No. 08/499,673 filedJul. 7, 1995, now U.S. Pat. No. 5,629,764, in the United States ofAmerica by RAMENDRA D. BAHUGUNA and THOMAS M. CORBOLINE entitled "APRISM FINGERPRINT SENSOR USING A HOLOGRAPHIC OPTICAL ELEMENT".

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to generation and capture of high contrast,detailed optical fingerprint images, and in particular, to an opticalsensor combining a right angle prism with a holographic phase grating toproduce a fingerprint image without dimensional spatial distortion andastigmatism. The invention further relates to a method of constructingthe holographic phase gratings for the invented fingerprint sensor

2. Description of the Prior Art

Total internal reflection has been used for a long time to opticallysense ridges and valleys of a finger surface, i.e., to optically capturea fingerprint. H. J. Caulfield and D. R. Perkins, (Caulfield et al) inU.S. Pat. No. 3,716,301 teaches the use of a prism sensor based on totalinternal reflection in their holographic finger print recognitionsystem. S. Igaki, S Eguchi, F. Yamagishi, H. Ikeda and T. Inagaki (Igakiet. al.) in a paper published in Applied Optics, Vol. 31, pp. 1794-1802(1955), disclose a parallel plate sensor using total internal reflectionin a flat glass plate to transport a captured fingerprint image incombination with a holographic grating both which retrieve the imagereflecting within the plate and which corrects to a degree fordimensional aberration in that image. The device described by Igaki et.al. works in a scattering mode rather than absorption. Accordingly,image contrast is reversed with respect to that obtained by Caulfield etal., and a pair of cylindrical lenses are required for correctingastigmatism in the captured image.

More recently, M. Metz, C. Flatow, Z. Coleman and N. J. Phillips (M.Metz et. al.) have developed an edge lit hologram for capturing afingerprint images not based on total internal reflection. [See LaserFocus World, May, 1994, pp. 159-163 and a paper entitled "The Use OfEdge-lit Holograms For Compact Fingerprint Capture" published in theConference Proceedings of "Card-Tech Secure-Tech 1995" held Apr. 10-13,1995, in Washington D.C., pp. 221-228.

The primary disadvantage of existing prism fingerprint sensors is thatthe fingerprint is compressed in one orthogonal dimension with respectto the other by a factor equal to the cosine of the angle at which theimage plane is inclined relative to the normal. To explain, any image ina plane viewed at an angle 45° to the normal is compressed in onedimension by cos. (45°), i.e. by 1/√2. [Right angle isosceles (45°)prisms are typically described for obtaining fingerprint images.]

Another serious drawback of existing prism fingerprint sensors is thatthe image plane of the fingerprint emerges from the various sensorsinclined with respect to the optical axis. Accordingly, it usuallynecessary to optically and/or computationally reorient the image to aplane normal (⊥) to the optical axis for optimal resolution.

The glass plate sensor of S. Igaki et al utilizes a holographic gratingto diffract internally reflected light propagating within a glass plateat an angle greater than the critical angle so that the wave can emergefrom the plate into the air. S. Igaki et al specifically point out thatto create such a grating, one of the two interfering light waves forconstructing the hologram must meet the conditions of total internalreflection, that is light cannot enter the holographic recording platefrom the air. S. Igaki et al then describe a complicated procedure forcreating a suitable holographic grating involving a different wavelengthof light. Finally, the glass plate holographic grating sensor describedby S. Igaki et al has astigmatism because the spherically divergentwaveforms scattered from a fingerprint ridges are diffracted by fringeplanes of a holographic grating creating by planar waveforms.

SUMMARY OF THE INVENTION

The invented fingerprint sensor includes a holographic phase gratingoptically coupled with and forming a right angle surface of a prismwhich diffracts light totally internally reflecting from its externalsurface to propagate normally (⊥) back into the prism. In areas wherethe ridges of the finger surface are in contact with the grating surfacethe illuminating light is refracted and absorbed. In areas correspondingto valleys and pores of the finger surface not in contact with thegrating surface, the illuminating light is totally internally reflected.The reflected light is diffracted by the holographic phase grating andemerges from the transmission surface of the prism producing a highcontrast, detailed image of the ridges, pores and valleys of the fingersurface sensor surface of the sensor oriented in a plane normal to theoptical axis. In particular, the holographic phase grating eliminatesimage distortion due to dimensional compression. And, because thewaveform of the image within and emerging from the invented sensor isplanar, there is no necessity for optically correcting for astigmatismbefore recording the image electronically or in film. Nor is there anynecessity for optically or computationally correcting or enhancing therecorded images thereafter.

A novel method for recording a holographic phase grating for theinvented sensor includes the steps of: (a) Sandwiching an unexposedholographic plate optically between a right angle (transmission) surfaceof a right angled prism and a blackened glass plate using a suitableliquid such as isopropyl alcohol; (b) Exposing the holographic plateusing a collimated laser beam split by amplitude division where a firstbeam is directed normally into a hypotenuse surface of the particularright angle prism, and a second beam is directed normally into the otherright angle (transmission) surface of the prism, the second beam beingtotally internal reflected from the internal hypotenuse prism surface topropagate normally (⊥) toward the holographic plate; and (c) Removing,developing and bleaching the exposed holographic plate to provide aphase grating. The incident beams within the holographic plate createfringe planes bisecting the angle between the respective incident beams.The blackened glass plate optically coupled to the exterior surface ofthe holographic plate precludes total internal reflection of the firstbeam directed normally through the hypotenuse surface of the prism atthat exterior surface of the holographic plate and absorbs/attenuatesthe constructing/interfering incident light beams transmitted by theholographic plate. The created holographic phase grating is then rotated180° in its plane and recouped (glued) to the same (sensor) right anglesurface of the particular or an identical prism to insure that lighttotally internally reflected from the exterior surface of theholographic phase grating is diffractied and directed perpendicularlyback into the body of the prism. The primary advantages of the inventedsensor derive from the fact that: (a) the emerging fingerprint imagesare free of dimensional distortion, i.e., are not elongated orcompressed in one orthogonal dimension with respect to the other in theplane of the image; and (b) the waveforms of the fingerprint image areplanar. Accordingly, fingerprint images generated from the inventedfingerprint sensor require neither optical nor computational correctionof dimensional aberration or astigmatism before capture or recording infilm or electronically.

An important aspect of the invented sensor is that both white light andlaser light, collimated or diffuse, (preferably filtered) can be used toilluminate the sensor face--finger surface interface through a prism.With diffuse laser-illumination, the diffuser should be rotated at arate sufficient to average speckling out.

A feature of the invented fingerprint sensor is that latent fingerprintsleft on the sensing surface of the prism from previous use of the sensorare hardly noticeable with a diffuse illumination light source.

Most importantly, the high contrast, detailed output images provided bythe invented fingerprint sensor are ideally suited for systemsrecording, recognizing and verifying fingerprints. In fact, fingerprintimages obtainable with the invented sensor are of such quality to allowtrue resolution and actual measurement of pore distributions on fingersurface ridges.

The quality of the images produced are also ideally suited for video andother electron scanning cameras both for real time display anddigitization.

Still other features, aspects, advantages and objects of and provided bythe invented combination of a holographic phase grating opticallycoupled to one surface of a right angle prism for providing highcontrast, detailed fingerprint images oriented normally (⊥) to anoptical axis utilizing total internal reflection phenomenon will becomeapparent and/or be more fully understood with reference to the followingdetailed explanation in context of drawings showing schematicembodiments of components of the respective optical elements thoughtnecessary for simply and reliably producing such images.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic illustration of typical prior art fingerprintsensor using a prism.

FIG. 2. is a schematic illustration showing the basic opticalconfiguration of the preferred version of the invented fingerprintsensor where the exterior surface of the holographic phase gratingfunctions as the sensor surface.

FIG. 3. schematically illustrates a preferred method for recording ofthe holographic phase grating.

FIG. 4. depicts the orientation of the diffracting fringe planes withinthe emulsion of the holographic phase grating after the grating has beenrotated 180° in its plane.

FIG. 5. illustrates factors affecting imaging of the ridges and valleysof a finger--surface interface.

FIG. 5A illustrates details of the finger--surface interface imageproduced by reflected and refracted light in the plane A--A.

FIG. 6. schematically illustrates an embodiment of the invented sensorusing diffuse light.

FIG. 7a. is a photograph showing a fingerprint image using collimatedillumination.

FIG. 7b. is a photograph showing the same fingerprint as in FIG. 8a.using diffuse illumination.

FIG. 7c. is a photograph showing a latent image of a fingerprint afterthe finger is removed from the sensor surface of the holographic phasegrating of the invented sensor using collimated illumination.

FIG. 7d is a photograph showing a hardly noticeable latent image of afingerprint after the finger is removed from sensor surface of theholographic phase grating of the invented sensor using diffuseillumination.

FIG. 8 illustrates the components of a typical fingerprint capture andrecording system utilizing the invented fingerprint sensor.

DETAILED DESCRIPTION OF PREFERRED AND EXEMPLARY EMBODIMENTS

Looking at FIGS. 1 & 8, the elements of a typical prior art fingerprintprism sensor 11 include a right angle (45°) prism 12 with a hypotenuseface A, a right angle light input surface B, and a right angle outputsurface C. As shown a finger is placed in contact with the exteriorsurface of the hypotenuse face A of the prism. The interface 13 betweenthe finger surface and the exterior hypotenuse surface A of the prism 12is illuminated by light from an exterior source (not shown), preferablycollimated. The light rays propagate normally into the right angle lightinput surface B of the prism 12 as indicated by the arrows 15. Theilluminating light is absorbed in areas where ridges on the fingersurface contact the hypotenuse face A and is totally internallyreflected in areas of the finger surface ridges not in contact with thesurface A, i.e. the valleys and pores of the finger surface. Thereflected light 16 propagates out the right angle output surface C ofthe prism, is imaged by the imaging lens and captured by an imagerecording mechanism CCD, (in this case a CCD photo sensor array or videocamera). As shown in FIG. 8, the captured image is processed (digitized)using a computer then compared using a corrolator with data availablefrom a reference bank to provide an indication of a match or mismatch.

Because the illuminated interface of the finger surface and hypotenuseface is inclined relative to the optical axis of the system, and therecording plane, the image captured from the right angle output surfaceC of the prism 12 is compressed in the (vertical) dimension of the rightangle prism surface C by a factor equal to the Cosine of the angle ofinclination of the image relative to the recording plane, or normal,i.e., in the example given by a factor f=cos. (45°)=1/√2 or 0.7071. Tooptimize resolution, the captured image can thereafter becomputationally corrected to eliminate the dimensional distortion.However, because such computational correction is typically performedafter the image is recorded, detail compressed into, for example, 0.707pixel, is effectively uniformly expanded into 1 pixel, when, in actualfact, such detail is not so uniformly distributed. This means that somedetail will be lost. Accordingly, while computational correction ofdimensional distortion of an image inclined relative to the recordingplane post recordation may improve resolution, it also introduces error.

It is also possible to optically reorient inclined light images beforerecording. However, such optical systems are elaborate, typicallyrequiring precision optical components and precise alignment. In thehurly burly world of capturing fingerprint images for recording,recognition and verification, the task of and the prospects formaintaining such precision alignment of multiple optical elements overtime are both daunting and unrealizable.

Looking at FIG. 2, the invented fingerprint sensor 17 also includes aright angle 45° degree prism 12 with a hypotenuse surface A, a rightangle sensor surface B and a right angle transmission surface C. Aholographic phase grating 14 is optically coupled to the right anglesensor surface B of the prism 12. The holographic phase grating 14 isoriented such that its diffracting fringe planes 16 diffract planarwaveform light totally internally reflecting from the interior surface18 of the grating 14 to propagate normally (⊥) back through the rightangle transmission surface B of the prism 12.

A suitable holographic phase grating for the invented fingerprint sensorcan be recorded in several ways. A effective, yet very simple method isillustrated in FIG. 3. A collimated laser beam (not shown) is split intotwo constructing laser light beams 31 & 32 by amplitude division. Onebeam 31 is directed to normally (⊥) illuminate the hypotenuse surface Aof the right angled prism 12 while the other beam 32 is directed tonormally (⊥) illuminate the right angle (transmission) surface C. Aholographic plate 34, e.g. an Agfa 8E75 plate for a He--Ne laser, isplaced and maintained in optical contact with right angle (sensor)surface B of the prism 12. A blackened bottom glass plate 37 is placedin optical contact with the exterior surface 38 of the holographic plate34. Layers of isopropyl alcohol 36 or other suitable liquids may be usedas an optically coupling medium between the respective surfaces. Theblackened bottom glass plate 37 eliminates total internal reflection ofconstructing beam 31 off the exterior surface 38 of the holographicplate 34. The holographic plate 34 is exposed to the two constructinglaser beams 31 & 32 for an amount of time to required produce an opticaldensity of 2. The plate 34 is then developed and bleached to provide aholographic phase grating 14. Care must be taken to ensure that theemulsion thickness of the plate 34 does not change in the chemicalprocessing.

As shown in FIG. 3, the interference fringe planes 39 created in thedeveloped holographic phase grating 14 incline to the left at an anglewhich bisects the angle between the respective incident constructingbeams 31 & 32. However, as shown in FIG. 5., to diffract light totallyinternally reflecting off the interior surface of exterior face 38 ofthe developed holographic phase grating 34 in the preferred version ofthe invented fingerprint sensor, [FIG. 2] directing it to propagatenormally (⊥) back into the right angle (transmission) surface B of theprism 12, the fringe planes 39 of the grating must be inclined to theright at an angle of 22.5° with respect to the normal. In particular,the developed holographic phase grating 14 must be rotated 180° (in itsown plane) because light reflecting from the interior face of exteriorsurface 18 is rotated 180° with respect to incident light beams 31 & 32that creating the phase grating 14. Upon rotation, the developedholographic phase grating 14 is glued to the right angle sensor surfaceB of that or an identical prism for the sensor shown in FIG. 2.

When constructing holographic phase gratings 14 for white light, aslight adjustment in the incident beam angle of the two constructinglaser beams 31 & 32 may be necessary in order to assure that, after thegrating is rotated and glued to the right angle sensor surface B of theprism, a selected or desired range of light wavelengths are diffractedperpendicularly back into the body of the prisim.

With the invented fingerprint sensor illustrated in FIG. 2 because theholographic grating 14 is physically secured (glued) to a right angle(transmission) surface B of the prism 12, mechanical shocks, vibrationand the like do not affect the optical orientation of the planarwaveform image emerging from the transmission surface C of the prism 12.Also, in order to assure optimal optical performance of the prism 12,holographic phase grating 14 combination, it is suggested that theparticular sensor prism 12 be used in creation of its particularholographic phase grating 14.

Turning now to FIG. 5, to generate a finger print image with theinvented fingerprint sensor a finger surface 61 is pressed against theholographic phase grating 14 as shown. Illuminating light rays 62 from acollimated white light source, or a laser (not shown) are directed innormal incidence to the hypotenuse surface A of the prism 12. If a laseris used, the illuminating light should be the same wavelength as used inconstructing the holographic phase grating 14. The rays 62 hit the prismsurface B at an angle of 45° and with minimal refraction propagate intothe holographic grating 14, ideally at the same angle (45°). Inparticular, for optimal results, the emulsion chosen to contain theconstructed holographic phase grating 14 should have a refractive indexthe same as or approximately the same as the refractive index of thematerial composing the prism 12. Accordingly, the rays 62 will totallyinternally reflected at the bottom surface of the holographic phasegrating 14 where the valleys 67 between and pores 68 [See FIGS. 7a & 7b]in the fingerprint ridges are located. However, light rays 62 incidenton the finger--surface interface 63 in areas where fingerprint ridges 69are in contact with the exterior surface 38 of the holographic phasegrating 14, refract through the finger--surface interface 63 and are inessence absorbed. The reflected rays 64 thus carry information from thefinger--surface interface 63 in as a high contrast planar waveformpattern, the valleys 67 and pores 68 appearing bright and the ridges 69dark. [See FIGS. 7a & 7b] The totally internally reflected rays 63containing the image are diffracted by the fringe planes 39 of theholographic grating 14 (see FIG. 3) and are directed to a normal (⊥)orientation with respect to prism surface B rotating the plane of thecontained image to an orientation parallel to that of the prism surfaceB. The reflected rays 64 after total internal reflection off theinternal surface A of the prism 12 and emerge out through its rightangle transmission surface C. The rays 64 containing the fingerprintimage are then collected by a video camera system 70 and projected via aTV monitor or input into a fingerprint recognition system for recordingand/or verification.[See FIG. 8.] Alternatively, the fingerprint imagecan be captured in film and thereafter reproduced photographically. [SeeFIGS. 7a-d.]

Turning to FIG. 6, for diffuse illumination, the invented fingerprintsensor includes a fine ground glass diffuser plate 71 placed in veryclose proximity (within a few millimeters or so) to the hypotenusesurface A of the prism 12. The ground glass diffuser plate 71 diffusesthe light from an incoherent white light source 72. The diffused lightrays 73 illuminates the interface 74 of the finger surface 76 and theexterior surface 38 of the holographic phase grating 14. As before, thediffused rays incident on the interface at angles greater than thecritical angle for total internal reflection are reflected except wherethe fingerprint ridges contact the surface 38. All other rays of thediffused light simply partially reflect from and refracts through theexterior surface 38. The refracted light exiting through the holographicphase grating 14 is lost. And, again the fringe planes 39 of theholographic phase grating 14 diffract the totally internally reflectedlight rays 64 rotating or redirecting them into normal (⊥) incidencerelative to the right angle (sensor) surface B of the prism 12. Thebalance of the reflected rays also diffracted by the holographic phasegrating 14 will be directed at angles other than normal incidencerelative to right angle sensor surface B of the prism 12. The redirectedtotally reflected rays 64 are again totally internally reflected fromthe internal hypotenuse surface A of the prim 12 and are directednormally (⊥) out the right angle (transmission) surface C of the prism.The balance of the light rays internally reflecting from the exteriorsurface 38 of the holographic phase grating 14 (unwanted light) willstrike the internal hypotenuse surface A and either refract out orinternally reflect and attenuate within the prism 12. In either case,such unwanted light rays are not aligned with the optical axis of theoptical system capturing the orthogonal rays 64 containing thefingerprint image exiting the invented sensor. If laser light is used asthe illuminating light source with a ground glass diffuser plate 71, theplate 71 must be rotated or oscillated at a speed high enough to averagespeckling out.

In particular, looking at FIG. 5, a motor or oscillator 77 ismechanically coupled to the diffuser plate 71, schematically representedby a dashed line, for rotating or oscillating the diffuser plate 71.

The quality of the fingerprint images captured by the inventedfingerprint sensor as evidenced by FIGS. 7a and 7b are extremely good;even the pores 68 on the fingerprint ridges 69 are quite distinct. Theimage quality seems to be best when the prism is illuminated withcollimated light. However, it should be noted that with collimatedillumination, the right angle grating surface 38 or the hypotenusesensor surface A of the prism 12 has to be thoroughly cleaned after eachuse to prevent latent fingerprints of previous users from obscuringlater images. (see FIG. 7c.) With a clean prism the resolution andquality of fingerprint images generated is excellent.

With diffuse light illumination the pores 68 on the fingerprint ridge 69are not quite as distinct, being slightly smeared along the direction ofthe finger 83. To explain, looking again at FIG. 5, the image of thefingerprint ridge 69 is shifted by a distance x. Because the image-shiftx is different for the different angles of light incident on the finger,a point on a ridge 69 will imaged as a small line parallel to thefinger. Such linear smearing is directly proportional to the thicknessof the holographic phase grating, and therefore, can be reduced by usinga thin hologram phase grating plates, preferably less than 0.3 mm thick.With a thin hologram phase grating 14, diffuse illumination can producefingerprint image quality approaching that obtained with collimatedillumination. Diffuse illumination has an added advantage in that thelatent prints of previous users are hardly noticeable (See FIG. 7d).Also, with diffuse illumination, the fingerprint can be viewed with thenaked eye without danger of eye damage over a large field of view. [Theimages presented in FIGS. 7a-7d are reproduced from actual photographsdeveloped from film capturing images produced by the inventedfingerprint sensor. These images demonstrate better than words candescribe high quality of fingerprint images that are obtainable usingthe invented fingerprint sensor.]

A ground glass diffuser plate 71 producing a diffuse light source forilluminating the finger surface--sensor surface interface also presentssubstantial advantages in fingerprint verification systems diagrammed inFIG. 8. In particular such systems typically utilize a convergingwavefront carrying information from the actual fingersurface which isoperated on by a holographic filter to reproduce an output beam similarto a reference beam used in creating the filter. The output beam isfocused by a lens and sensed a photo detectors. The magitude of thesignal produced by the dectector indicates a match or mismatch. (SeeCaulfield et al, and U.S. Pat. Nos. 5,138,468 & 5,095,194 J. Barbanell)

An observed Achilles heel of such verification systems is that cleaningthe input or sensing interface surface of the fingerprint sensor with aliquid, such as isopropyl alcohol, often leaves a distribution ofstreaks on the input sensing interface. The streaks diffract theilluminating collimated light flux into diffraction patterns which arecollected and directed along the verification optical path of thesystem. Because both the flux and incidence of such diffraction patternsare distributed across the entire field of view operated on by theholographic Fourier-transform filter elements, there is a relativelyhigh probability, that output reference beams of the filter element maybe reproduced triggering a false verification.

In contrast, when the light illuminating such liquid-air interface at asensor surface is diffused, the streaks are smoothed out reducing anglesof diffraction to very low values. In other words, the diffuse lightrays diffracted by streaks are concentrated along the optical axis ofthe system which typically is blocked at the hologram plane,Accordingly, the chance of a false verification is essentiallyeliminated.

To a degree, the above described phenomenon is also illustrated by thephotographs presented as FIGS. 7c and 7d which depict latent fingerprintimages picked up from the sensor surface of the preferred version of theinvented sensor after the finger has been removed. In particular, thelatent fingerprint image of FIG. 7c produced with collimatedilluminating light source has greater contrast than that of FIG. 7dproduced with diffuse light.

The invented fingerprint sensor and method for constructing theassociated holographic phase grating has been described in context ofboth representative and preferred embodiments. There are manymodifications and variations can be made to the invented stop which,while not exactly described herein, fall within the spirit and the scopeof invention as described and set forth in the appended claims.

We claim:
 1. A method for recording a planar holographic phase gratingplate for diffracting and redirecting light totally internallyreflecting from a first face perpendicularly back through a second faceparallel the first face comprising the steps of:a) sandwiching anunexposed holographic plate having first and second parallel planarfaces optically between one right angle transmission surface of a rightangled prism having a hypotenuse surface and two right angle surfaces,and a blackened glass plate using a suitable liquid optical couplingmedium including isopropyl alcohol, with the second planar face adjacentthe right angle surface of the prism and the first planar face adjacentto the blackened glass plate; b) exposing the holographic plate using acollimated coherent light beam split by amplitude division into a firstconstructing light beam directed normally into the hypotenuse surface ofthe right angle prism for refracting into the holographic plate at anangle of refraction θ greater than a critical angle for total internalreflection from the first face of the holographic plate, and a secondconstructing light beam directed normally into the remaining right anglesurface of the prism for totally internally reflecting off thehypotenuse prism surface to propagate toward normal (⊥) incidence withand into the holographic plate for generating fringe planes within theplate oriented at an angle bisecting the angle between the respectiveconstructing beams within the holographic plate; (c) removing,developing and bleaching the exposed holographic plate to provide aplanar holographic phase grating; and (d) rotating the holographic phasegrating 180° in a plane parallel its respective faces, whereby, lightrefracting into the second face of the developed holographic phasegrating in the same direction as the first constructing light beam at anangle greater than the critical angle for total internal reflectionreflects internally from the first face of the holographic phase gratingand is diffracted by the fringe planes to propagated perpendicularlyback out its second face.
 2. The method for recording a planarholographic phase grating plate of claim 1 wherein the holographic plateis exposed to the first and second constructing light beams for anamount of time to required produce an optical density of
 2. 3. Themethod for recording a planar holographic phase grating plate of claim 1wherein the right angle prism, the unexposed holographic plate and theresulting holographic phase grating all have an approximately equalindex of refraction.
 4. The method for recording a planar holographicphase grating plate of claim 1 wherein the unexposed holographic plateand the resulting planar holographic phase grating have an approximatelyequal thickness that is less than 0.3 mm.
 5. A method for recording aplanar holographic phase grating plate for diffracting and redirecting aselected range of light wavelengths of white light totally internallyreflecting from a first face back through a second face parallel thefirst face in a selected direction comprising the steps of:a)sandwiching an unexposed holographic plate having first and secondparallel planar faces optically between one right angle transmissionsurface of a right angled prism having a hypotenuse surface and tworight angle surfaces, and a blackened glass plate using a suitableliquid optical coupling medium including isopropyl alcohol, with thesecond planar face adjacent the right angle surface of the prism and thefirst planar face adjacent to the blackened glass plate; b) exposing theholographic plate by:(i) producing a collimated coherent light beamusing a laser; (ii) splitting the collimated coherent light beam byamplitude division into a first constructing light beam, and a secondconstructing light beam; (iii) directing the first constructing lightbeam into the hypotenuse surface of the right angle prism for refractinginto the holographic plate at an angle of refraction θ greater than acritical angle for total internal reflection from the first face of theholographic plate; (iv) directing the second constructing light beaminto the remaining right angle surface of the prism for totallyinternally reflecting off the hypotenuse prism surface to propagatetoward approximate incidence at the selected direction with and into theholographic plate; (v) adjusting respective angles at which the firstand second constructing light beams are directed into the hypotenuse andright angle surfaces of the prism for generating fringe planes withinthe holographic plate at a particular angle φ chosen for diffracting theselected range of light wavelengths; (c) removing, developing andbleaching the exposed holographic plate to provide a planar holographicphase grating: and (d) rotating the holographic phase grating 180° in aplane parallel its respective faces, whereby, the selected range oflight wavelengths of white light refracting into the second face of thedeveloped holographic phase grating in the same direction as the firstconstructing light beam at an angle greater than the critical angle fortotal internal reflection reflects internally from the first face and isdiffracted by the fringe planes to propagate in the selected directionback out the second face.